1) Field of the Invention
The present invention relates to an optical switch and a method for controlling a drive voltage thereof.
2) Background of the Invention
Developing an optical switch, which is formed from a light deflection device using a crystal involving an electro-optic effect, as an optical switch applied to an optical communications system has recently been pursued.
As shown in FIG. 14, a light deflection device 101 has a ferroelectric crystal 102 possessing an electro-optic effect, and a pair of electrodes 103 formed on opposing faces (a front face 102-1 and a back face 102-2 opposing with each other in the drawing) of the crystal 102. When a drive voltage is supplied to the electrodes 103, light entering one side face 102-3 of the crystal 102 can be deflected by means of the electro-optic effect.
This light deflection device 101 has the property of varying a deflection angle of input light by means of varying the drive voltage supplied to the electrodes 103. Specifically, as shown in FIG. 14, the light deflection device 101 is configured so as to be able to deflect and output the input light in a deflected direction D1 by means of setting the drive voltage V to V1; to deflect and output the input light in a deflected direction D2 by means of setting the drive voltage V to V2; and to deflect and output the input light in a deflected direction D3 by means of setting the drive voltage V to V3.
By means of setting an output port to a light output position which varies according to a deflection angle of the input light, the light deflection device 101 can be configured as an optical switch 101A which selectively switches the output destination of the input light to any one of a plurality of output ports (three output ports #21 to #23 in the drawing) by means of switching the drive voltage.
Moreover, in this optical switch 101A, the drive voltage supplied to the electrodes 103 is set so as to be switched according to an output port which is the output destination of the input light. Accordingly, the optical switch 101A can be equipped with a control circuit 104 and a drive circuit 105. The control circuit 104 is made to be able to fetch data pertaining to a drive voltage to be used for setting a deflection angle for each output port which is a destination, and to output the thus-fetched data as a control signal to the drive circuit 105. The drive circuit 105 can supply the electrodes 103 with a drive voltage in accordance with the control signal that has been output from the control circuit 104.
Known documents relating to the present invention are Patent Documents 1 to 6 provided below.                (Patent Document 1) JP-2002-318398A        (Patent Document 2) JP-2003-185984A        (Patent Document 3) JP-2000-114629A        (Patent Document 4) JP-2000-269892A        (Patent Document 5) JP-HEI-7-212315A        (Patent Document 6) JP-HEI-10-228007A        
However, the optical switch 101A formed from such a light deflection device 101 can speed up the switching response of an output channel, by means of the control circuit 104 and the drive circuit 105 controlling the supply of the drive voltage. Conversely, the drive circuit 105 must be provided with a comparatively-wide dynamic range, as the range of the drive voltage, depending on the position of the output port. For instance, depending on a deflection direction, the value of a drive voltage to be supplied to the electrodes 103 must be increased to about hundreds of volts, which is tens of times or more the voltage value of the control signal output from the control circuit 104; or the voltage must be substantially cut off.
When the drive voltage is increased to about tens of times or more the voltage of the control signal as mentioned previously, a high voltage output circuit must be configured as the drive circuit 105 to thereby generate a drive voltage whose value is tens of times the voltage of the control signal. The value of the drive voltage generated by such a drive circuit 105 contains an error component in relation to the control signal output from the control circuit 104. Accordingly, difficulty is encountered in controlling a deflection angle with comparative precision by means of the control circuit 104 controlling the drive voltage output from the drive circuit 105, to thus raise a problem of occurrence of losses in the power of light output from the output port.
As a result of the light deflection device being used as the optical switch 101A, a relationship between the drive voltage output from the drive circuit 105 and the deflection angle gradually changes [i.e., occurrence of a DC (Direct Current) drift], which also raises a problem of the DC drift hindering stabilization of power of the output light.
As shown in FIG. 15, in a case where V2, which is set when the output port #22 is taken as a destination, is supplied as a drive voltage, losses in the light to be output from the output port #22 (optically coupled to the output port) is minimized before occurrence of a DC drift, and hence the power of the light can be maximized. When the drive voltage V2 is supplied after occurrence of a DC drift, a deviation Δθ arises in the angle at which input light is deflected. Losses arise in the power of light output from the output port #22.
A conceivable principal factor responsible for this DC drift is the distortion remaining in an atomic arrangement constituting a crystal structure, because of an increase in an accumulated time during which a comparatively-high drive voltage is supplied to the crystal 102, even when the drive voltage is cut off.
As mentioned previously, Patent Documents 1 to 6 described above do not provide any technique of controlling a deflection angle with comparatively high accuracy while covering the range of a comparatively-wide deflection angle for the purpose of switchingly setting an output port as a deflectable range, or a technique of stabilizing the power of an output in the event of occurrence of a DC drift while maintaining the performance of a response to switching of an output channel.